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POLYMER 'WORMS' LINE UP ON SILICON New method uses cylindrical polymer micelles to create nanoscale features RON DAGANI To improve the speed and performance of integrated-circuit chips, manufacturers are continually trying to cram more devices on them. This requires that the devices become smaller--that is, have finer features. But as the smallest dimension of these features shrinks below 100 nm, it becomes more and more difficult to form patterns using conventional lithographic techniques. WORMLIKE Cylindrical micelles (top) composed of a ferrocenylsilane-siloxane block copolymer are induced to assemble on a silicon surface to form linear features. The micelle lines are then transformed into a pattern of ceramic nanolines, seen in the scanning force micrograph (bottom). Many scientists are exploring alternative approaches in which nanoscale features are built up from smaller nanostructures. The latest variation on this theme--and a particularly clever one at that, according to one chemist--comes from a collaborative team of researchers in Canada and Germany. They have used cylindrical polymeric micelles as building blocks to fabricate ceramic lines on a semiconductor surface [J. Am. Chem. Soc., 123, 3147 (2001)]. The micelles are made of a block copolymer containing substituted ferrocenylsilane and siloxane units. This copolymer was first synthesized a few years ago by chemistry professor Ian Manners' group at the University of Toronto. Manners, fellow chemistry professor Mitchell A. Winnik, and graduate student Jason A. Massey found that when this copolymer is dissolved in n-hexane, the strands assemble themselves into stable cylindrical micelles. In these micelles, the polyferrocenylsilane segments form an iron-rich core surrounded by an insulating polysiloxane sheath. The micelles, which are flexible and about 20 nm thick, can be made to any length from about 70 nm to longer than 10 mm. Because these wormlike micelles contain significant amounts of iron, silicon, and oxygen, it occurred to the Toronto researchers that these nanostructures potentially could be converted into magnetic ceramic nanopatterns. But to accomplish this, they had to find a simple way to orient the micelles into continuous lines. Manners and Winnik collaborated with chemist Martin Möller, physicist Joachim P. Spatz, and coworkers at the University of Ulm, adopting a procedure Möller's group developed for use with spherical micelles. In this procedure, the researchers first coat a silicon substrate with a layer of a plastic resist. Linear grooves that expose the silicon surface are then carved into this resist using an electron beam and standard etching chemistry. The substrate is then coated with a hexane suspension of 500-nm-long micelles, and the hexane is evaporated. If the micelle concentration is carefully adjusted, capillary forces will ensure that the micelles that end up in the grooves of the resist will line up end to end along the edges of the groove, forming single lines on the silicon. Any micelles that end up on top of the resist will be washed away when the resist is removed with acetone, Manners explains. The lines of micelles that have formed on the silicon surface remain there, however. The substrate surface is then exposed to a hydrogen plasma, which burns off most of the micelles' organic material. Left behind is a ceramic residue--essentially, an iron-silicon oxide--in the form of nanoscale lines. These are about 4 nm high, less than 10 nm wide, and several micrometers long. Manners hopes that these ceramic lines will display magnetic and/or conductive or semiconductive properties, although his coworkers haven't yet made the relevant measurements. Also, the ceramic lines potentially could serve as etching resists for producing two-dimensional quantum wires in semiconducting substrates, he says. The preliminary results reported by Manners, Winnik, and Möller are merely a proof of concept. Although this approach offers possibilities for making small features on chips, Manners says, "a tremendous amount of work" still needs to be done to prove the concept is practical and commercial. From a fundamental standpoint, though, these results are "absolutely spectacular," gushes Uwe H. F. Bunz, an associate professor of chemistry at the University of South Carolina, Columbia. He believes that this approach, which combines "top-down" (lithographic) and "bottom-up" (nanotechnological) techniques, should make it relatively easy to fabricate these types of nanowire structures. "I think it's a beautiful paper," Bunz remarks |
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